Rockwell Hardness Testing: Principles, Applications, and Limitations

Rockwell Hardness Testing: Principles and Method (HR)

The Rockwell test measures the permanent indentation depth under a controlled load in two steps:

1. Minor load (preload): An initial force of 3–10 kgf establishes the “0” reference point and minimizes the effect of surface irregularities.
2. Major load: An additional force (15–3,000 kgf, depending on the scale) is applied and then removed while the minor load remains. Hardness is calculated from the difference in depth before and after the major load.

Calculation formula:

Where:

• N and h: a scale-dependent constant.
• d: depth difference (mm).

Scales and Indenters

Rockwell uses more than 30 scales, divided into two groups:

• Regular scales (HR):
  • HRC: Diamond indenter, 150 kgf load (hardened steel, e.g. 60–65 HRC).
  • HRB: 1/16-inch carbide ball, 100 kgf load (soft metals such as aluminum and copper).
• Superficial scales (HRN/HRT):
  • HR15N: Used for thin materials or surface-hardened layers.

In Vietnam, the national hardness measurement standard approved by the Prime Minister specifies hardness measurement in HRC.

HRC hardness unit (Hardness Rockwell C) is used to express the hardness of materials such as SKD11, SKD61, SCM440, and DC11 steel. A Rockwell hardness tester uses the C scale (black lettering), a diamond cone indenter, and a 150 kg force. Scale C measures materials of medium to high hardness.

Note: Select a scale that matches the material, thickness, and hardness range to avoid measurement error.

Applications

• Quality control: Quickly verify material consistency in automotive, aerospace, and tool manufacturing.
• Material selection: Assess load-bearing performance using the relationship between hardness and tensile strength.
• Heat-treatment inspection: Confirm that steel has been hardened according to the required process.

Advantages

• Fast: Results are available in seconds, making the method suitable for mass production.
• Simple sample preparation: No surface polishing is required.
• Non-destructive: It leaves only a small indentation.
• Versatile: Suitable for metals, plastics, and composites.

Limitations

• Surface effects: Oil, dust, or curved surfaces can distort results.
• Scales are not directly comparable: Hardness values cannot be compared directly across scales.
• Indenter wear: A diamond indenter may lose accuracy over time.
• Thickness limit: The sample must be at least 10 times as thick as the indentation depth.

Recent improvements

• Automation:Automated testers (for example, Affri Dako Jominy) integrate a touchscreen and automatic load changes.
• Portable equipment: Enables on-site testing of large components.
• High accuracy: Dual load sensors and advanced software reduce measurement error.

Comparison with Other Methods

Best Practices

• Calibrate regularly with a reference block.
• Clean and flatten the sample surface.
• Follow the load dwell time to avoid deformation.

Hardness Conversion Table

Featured Rockwell hardness testers hardness testers

AFFRI hardness tester series – Italy

An automatic hardness tester for Rockwell and Superficial Rockwell methods, suitable for testing both large and small samples.

A semi-automatic Rockwell hardness tester for large components, delivering highly accurate measurements.